The prior art includes a number of arrangements for monitoring a vehicle's motion or position using light beams, infrared beams, ultrasonic beams, microwave beams, etc., coupled with a variety of signalling means. The following patents disclose such arrangements:
U.S. Pat. No. 4,808,997 to Barkley shows an arrangement including a photoelectric device for signalling the position of a vehicle. Briefly, a photoelectric beam is either reflected by a vehicle intercepting the beam or interrupts a beam reflected back to the transmitter by a mirror to signal the presence of the vehicle in the beam path.
U.S. Pat. No. 5,208,586 to Friberg discloses an arrangement for signalling when a vehicle has reached a predetermined position in a garage using an ultrasonic pulse generator and receiver and complex control circuitry.
U.S. Pat. No. 4,665,378 to Heckethorn discloses vehicle garaging methods and apparatus including a concentrated light beam which illuminates a spot on the floor when no vehicle is present and illuminates a spot on the dashboard (for example) when the vehicle is properly parked.
U.S. Pat. No. 5,128,670 to Jackson discloses a signalling system for intersecting railroads. In accordance with the Jackson arrangement, a light beam is projected across a roadway at a receiver. As a car breaks the beam, a caution signal is triggered to alert the driver that he or she is approaching an intersection.
U.S. Pat. No. 3,840,848 discloses a traffic monitoring arrangement including infrared transmitters for measuring the distance between vehicles. If a second vehicle is traveling too close to a first vehicle, a roadside sign is lighted warning the driver of the following vehicle of the hazardous practice.
All of these arrangements involve relatively high cost or complex positioning strategies. They are not easily translated to home use and/or multicar garages.
Less sophisticated prior art arrangements, however, which are suitable for home use are less reliable. Mechanical setups, such as compressed air hoses which trigger a signal responsive to the physical pressure of a vehicle on the hose or weights hung from the ceiling which the driver can see or which touch the vehicle when the vehicle is in the proper position can easily be moved out of position. None of the prior art arrangements combine the precision of the more sophisticated arrangements with the low cost and versatility of the less sophisticated arrangements.